Elimination reactions vicinal dihalides

Alkynes are prepared by elimination reactions, as discussed in Section 8.10. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne by two... 

After the first elimination of HC1, the reaction may be repeated on the alkene to give the alkyne. If, instead of a geminal dihalide, a vicinal dihalide is used, e.g. RCH2CHC1-CH2C1, then the same... 

At this point, you know only one way to synthesize a ketone—the addition of water to an alkyne (Section 6.6). The alkyne can be prepared from two successive E2 reactions of a vicinal dihalide, which in turn can be synthesized from an alkene. The desired alkene can be prepared from the given starting material by an elimination reaction. 

OTHER 1,2-ELIMINATION REACTIONS Dehalogenation of Vicinal Dihalides... 

In each of the examples of 1,2-elimination reactions discussed above, one of the groups eliminated was a proton. Another iype of 1,2-elimination reaction is the dehalogenation of vicinal dihalides. Iodide ion can be used as the dehalogenating agent in both protic and aprotic solvents (equation 10.40). 

General Preparations.—Alkenes. Dehalogenation of vicinal dihalides is easily carried out using sodium in liquid ammonia, e.g. 1,2-dichlorocyclo-octane gave ds-cyclo-octene in 95% yield.The scope of the synthesis of ap-unsaturated carbonyl compounds by syn-elimination from 2-phenylselenoxides has been studied only low yields of the desired ketone were obtained from 2-phenylseleno-cycloheptanone and cyclo-octanone because of the competing Pummerer rearrangements, but improved yields could be obtained by prior conversion of the ketones into ketals. The reaction worked well for 2-methoxycarbonyl cyclic ketones, e.g. 2-methoxycarbonylcyclo-oct-2-en-l-one was prepared in 93% yield. ... 

The mechanism is shown in Scheme 10 and involves reductive elimination of bromide anions to give quinodimethane intermediates (19) that polymerize. In the case of o-quinodimethane, their intermediacy has been proved by trapping via Diels-Alder reaction with a reactive maleic anhydride derivative [63, 64]. The second elimination step is analogous to the cathodic eliinination of vicinal dihalides, which is known to give tra s-alkenes [65]. 

Elimination reactions have been induced under phase transfer conditions by a variety of techniques [1, 2, 4, 5, 25—28]. Predominant among these is the dehydrohalogena-tion of alkyl halides or sulfonates [1, 2, 5] and the dehalogenation of vicinal dihalides [25]. In the former case, some elimination is observed as a side reaction of the intended Sn 2 halogen exchange reaction. The direct displacement of halide by halide (the... 

Alkynes can be prepared by the elimination of HX from alkyl halides in much the same manner as alkenes (Section 8.1). Treatment of a 1,2-dihaloalkane (a vicinal dihalide) with an excess amount of a strong base such as KOH or NaNH2 results in a twofold elimination of HX and formation of an alkyne. As with the elimination of HX to form an alkene, well defer a full discussion of this topic and the relevant reaction mechanisms until Chapter 11. 

Alkenes may be made from saturated compounds by various -elimination reactions. A vicinal dihalide may be dehalogenated by sodium iodide or activated zinc to give a double bond specifically between the carbons that... 

The interrelation among homovalent and ambivalent reactions on the five-atom pericycles (equations 3 and 4) was described and given a similar but more complex nomenclature reflecting their lower symmetry. Homovalent pericycles include the amine oxide eliminations in Scheme 1 and the sulfoxide-sulfenate rearrangement in Scheme 2, with the shells in boldface as in Figure 1. For the ambivalent reactions of equation (4) the ambivalent atom X is often not carbon, as seen in Scheme 3 for a metal reduction of vicinal dihalides (which may not be pericyclic) Scheme 3 has an unchanging shell of only one bond. The cycloaddition of sulfur dioxide to dienes in Scheme 4 is another with a three-bond shell. Numerous examples were quoted, again many not confirmed as pericyclic. ... 

Some synthetic methods introduce the carbon-carbon triple bond by functional group transformations such as the elimination reaction of vicinal dihalides. Other methods construct complex alkynes by forming carbon-carbon single bonds between two molecules, one of which already contains a triple bond. We discuss both synthetic methods in this section. 

The synthesis of an alkyne from either a geminal dihalide or a vicinal dihalide requires a strong base to eliminate two moles of hydrogen hahde. Two suitable reagents for this reaction are sodium amide in hquid ammonia as the solvent and potassium tert-butoxide in dimethyl sulfoxide. 

---